In the cases considered herein, the maximum temperatures in an ORC system are typically in the range from 330 to 380° C., although lower or higher temperatures are possible depending on the working fluid used in each individual case, such as a silicone oil, an aromatic hydrocarbon or the like.
The minimum temperature of the Rankine cycle depends on the cold source available to condense the working fluid. In the discussion that follows, mention will be made, for example, to a cold source in the form of cooling water which can be made available by a cooling tower, thus having a minimum temperature of around 25 to 30° C. and a flow rate such as to reach a typical temperature increase of around 10° C. on extracting heat from the cycle. However, the following considerations also apply to different cold sources, provided that the temperature difference between the maximum temperature of the available hot source and the maximum temperature of the cold source is high, say above 300° C.
FIG. 1 of the accompanying drawings shows a typical arrangement of an ORC system 100 adapted for the above-mentioned conditions and basically comprising:
a thermal source S1 for heating a vector fluid;
a primary circuit 10 in which flows the vector fluid coming from and returning to the thermal source S1 in the direction of the arrow F, F′, circulating by means of at least one recirculation pump—not shown in the Figure;
a heat exchange group ST1 which can include a super-heater 11, an evaporator 12 and a pre-heater 13 for the exchange of heat between the vector fluid and a working fluid circulating in a relative circuit 14 by means of at least one relative pump 15;
an expander 16, typically composed of a turbine assembly, fed by the working fluid in output from the heat exchange unit and usually followed by
a regenerator 17 and
a condenser assembly 18.
In an ORC system as shown in FIG. 2 on the Entropy (S)-Temperature (T) thermodynamic plane, the points indicated, which correspond to the same points in the layout diagram in FIG. 1 also, have the following meaning:
1. pump (15) input;
2. pump (15) output and start of regeneration;
3. end of regeneration (17, liquid side);
4. end of pre-heating (13);
5. end of evaporation (12);
6. end of superheating (11)/expander (16) input;
7. expander (16) output/regenerator (17, vapour side) input;
8. regenerator (17) output/condenser (18) input; and
9. start of condensation.
FIG. 3 shows the heat exchange diagrams for the exchangers introducing and extracting heat, respectively from the hot source (line 10, 11, 12, 13)—i.e. with respect to the heat exchange unit 11-13 and towards the cold source (line 14,15), i.e. the condenser 18.
Then, FIG. 4 shows a diagram related to the thermal exchange within the cycle, which occurs in the regenerator component. The thermal exchange phenomena are shown on the Power Exchanged (Q)—Temperature (T) plane.
The fact that the maximum and minimum temperatures of the cycle differ considerably from each other as a result of the great difference between the temperatures of the sources, ensures that the amount of thermal energy for each mass unit of fluid flowing through the machine, and that has to be exchanged in the regenerator, is very high. For many fluids, the ratio between the thermal energy exchanged at the regenerator and the energy entering from the external hot source is greater than one unit. Furthermore, the difference in thermal capacity between the liquid branch and the vapour branch of the regenerator is also considerable, albeit to a different extent depending on the working fluid used.
Consequently, even when a regenerator with a high thermal exchange capacity is used, i.e. a regenerator with a large surface area, in which the product of the exchange surface area and the thermal exchange coefficient is such as to result in a modest temperature difference between liquid and gaseous form on the lower-temperature side of the regenerator, on the other side of the regenerator the difference in temperature remains considerably greater.
By way of example, a modest value in the difference in temperature on the cold side of the regenerator, ΔTF=T8−T2 (FIG. 4), can typically be quantified as 15° C., while on the other side of the regenerator, the difference ΔTC=T7−T3 is 2 or 3 times greater.
In order to avoid this problem, the solution of drawing off part of the flow rate from the liquid branch is adopted, the drawn-off flow rate being heated up to a temperature close to the end-of-regeneration temperature of the remaining flow rate by means of an external thermal source. This solution, sometimes referred to in the art as “splitting”, is particularly advantageous when a thermal source is available that is characterized by a lower temperature than the main source.
However, there are systems where, apart from the main source, no high-temperature source is present or available, and the cold source is characterized by a relatively low temperature.
For example, this is the case of a system as schematically illustrated in FIG. 5, in which the only hot source available is a thermovector fluid which is heated in a bank of cylindrical-parabolic solar collectors 20 and which is supplied to the ORC system 100 via a feed conduit 21 and a return conduit 22 from/to the bank of collectors 20, possibly in the presence of a heat storage system 23 made according to known techniques.
As a cold source, the ORC system 100 uses a water flow supplied by a feed conduit 24 and a return conduit 25 from a cooling tower 26. In this example, the hot thermovector fluid may be a diathermic oil, i.e. a molten salt.
Nowadays, in several systems with a bank of cylindrical-parabolic collectors supplying systems that use the Rankine cycle with water vapour, rather than systems that use an organic fluid as working fluid, the thermovector fluid comprises a mixture of diphenyl and diphenyl oxide known under the trade name “Therminol VP1”.